Audio frequency doubler



AUDIO FREQUENCY DOUBLER Filed Jan. 29, 1940 INVENTOR. .4 00/314 .THOMPS 0N BY f n/a' ORNEYS.

Patented July 15, 1941 IAUDIO FREQUENCY DOUBLER Louis A. Thompson, Lakewood, Ohio, assignor to Acme Newspictures, Inc., New York, N. Y., a

corporation of Delaware Application lanuary 29, 1940, Serial No. 316,150

1 Claim. (01. 250-36) This invention rel-ates to a new and improved circuit for controlling or effecting audio frequency, and more particularly to an audio frequency doubler.

Although frequency doubling is relatively old, it has usually been accomplished by the selection of a harmonic of the fundamental frequency and amplifying the harmonic. This has its disadvantages in that certain circuit components such as filters must be provided to eliminate the fundamental as well as other filters to prevent the amplification of still higher undesirable harmonics. A still further disadvantage is realized in the loss of efficiency due to the discarding of the fundamental which makes it necessary to amplify the comparatively weak harmonic and thus requires more equipment for such amplification.

By the present invention I am able to double the frequency with a minimum of circuit components and apparatus and utilize the full efficiency of the fundamental frequency without loss. Still another distinct and desirable advantage resides in the ability to exactly double the frequency and obtain the correct phase relationship between the fundamental and the doubled frequency.

Although the particular circuit shown has a very useful application in the field of facsimile transmission, it is to be understood that the circuit is not limited to use of facsimile equipment, but may be put to many other uses and used in other combinations which will be obvious to those versed in the art. In connection with the use of the frequency doubler of my invention in facsimile transmission the carrier frequency used in transmitting pictures over the telephone lines must be within the frequency range of the telephone lines. The carrier frequency mostly used for picture transmission work is usually 1800 cycles. This may not, however, be the most desirable frequency to use for the reproduction of the pictures, because it limits the speed at which the picture can be transmitted with good quality. That is, the picture must be transmitted and received slowly because of the telephone line limitations and the pattern caused by the carrier may, when the picture is reproduced in the half-tone, show a still more distinct and undesirable pattern due to the periodic registration of the facsimile pattern with the half-tone screen. By the present invention the carrier of 1800 cycles best adapted to the telephone transmission characteristics is sent over the line and then with the apparatus of my invention, the

carrier frequency is doubled and amplified and furnishes a pattern formation so fine that it has substantially no such cooperative effect with the half-tone, eliminating the pattern effect normally found. I

The invention will become more apparent from the following description which is illustrated by the accompanying diagram showing a circuit embodying my invention.

As best shown in the figure the signal received from the telephone line and which may or may not be amplified prior to the use of the apparatus being described, is passed into the primary I of an audio frequency input transformer from which it is transferred inductively to the center tapped secondary 2. The end terminals 3 of the secondary are connected through adjustable rheostats 4 to the plates and grids 5 and 6 of a type 6-A-6 tube connected as a full wave rectifier. The center tap of the secondary is grounded at 8. The double cathode output 9 of the tube is connected by lead [0 to the input ll of a type 83 tube arranged as a class B amplifier.

The cathode I2 of the tube 89 is grounded. The 7 output I3 of the tube is connected by a lead I4 through a coupling condenser Hi to the primary l6 of an audio frequency output transformer, the other end of the primary being grounded. Plate voltage for the B9 tube is fed through an impedance network comprising inductance l9 and condenser 20 so arranged that the D. C.

plate current is kept out of primary of the output transformer. The secondary ll of the output transformer may be connected to whatever apparatus is desirable. The operation of the circuit is as follows.

A signal, which may be an 1800 cycle signal,

where it is transferred inductively to the secondary and is rectified in the 6-A-6 tube. The output of the tube is in the form of a 3600 impulse D. C. signal. The adjustable resistors 4 in the input transformer secondary provide means whereby the impulses may be regulated to provide impulses of equal amplitude on the grids of the G-A-G tubes. D. C. output of the rectifier is fed into the 89 tube which operates as a class B amplifier. Here the voltage is amplified and is fed into the output circuit. In the output circuit the A. C. component" is separated from the D. C. plate voltage.

When the carrier of 1800 cycles is amplitude modulated by the. picture signal, the output realized from the device is double frequency, but has the same modulation time constant.

The 3600 impulse This eliminates the pattern effect due to the combined effects of the low frequency carrier and the half-tone screen.

Although I have illustrated and described the apparatus as doubling the frequency only, it will be obvious that with other stages in cascade the frequency can be again doubled to provide a frequency of four times the original frequency and that the number of times the frequency can be doubled is not to be limited by the description.

Having thus described my invention, I claim:

An audio frequency doubler circuit comprising a transformer having a primary for connection to a source of audio frequency, a center tapped secondary for said transformer, a full wave rectifier having plates and a cathode, means connecting the secondary of the transformer between said plates, resistance means inserted in said plate circuit and adjustable to balance the input to said rectifier, an amplifier arranged to have its output current proportional to the amplitude of the input current and having at least a grid and cathode, the grid of said amplifier being connected to the cathode of said rectifier and the center tap being connected to the cathode of said amplifier, said amplifier being controlled by the rectifier to amplify the output thereof, means in the output circuit of the amplifier to supply the other half of an alternating Wave to said amplified output comprising an output transformer capacitively coupled to the plate circuit of the amplifier and tuned to the output frequency of the amplifier, means for supplying the plate voltage to the amplifier having a high impedance to the alternating current component of the plate voltage including an impedance coupled to the plate circuit ahead of the capacity coupling to the output amplifier.

LOUIS A. THOMPSON. 

